Steering valve with closed center

ABSTRACT

The invention concerns a steering valve with a closed center for supplying an actuator with hydraulic pressure, the steering valve comprising an input shaft, and output shaft, a torsionally resilient element which is connected at one end to the input shaft and at the other end to the output shaft, a control pressure switching arrangement and a torque/axial force conversion unit. A spring loaded seat valve is located in a bore of the input shaft, and closes a fluid passage passing through the torsion tube. The torque/axial force convertor opens the seat valve when relative twisting of the input shaft with respect to the valve casing occurs. According to the invention, in order to improve the response dynamics of such a steering valve, the valve comprises at least one axial bore for supplying hydraulic pressure. At one end this bore is closed by a valve which is loaded by the torque/axial force conversion unit against the hydraulic pressure.

BACKGROUND OF THE INVENTION

The invention pertains to a steering valve with a closed center forsupplying a control element with hydraulic pressure, consisting of aninput shaft, an output shaft, a torsionally elastic element that isconnected at one end to the input shaft and at the other to the outputshaft, a control pressure switching device and a torque/axial forceconverter unit.

Steering valves of this class are utilized in particular in powersteering systems. The structure of such power steering systems isconventional. An input shaft connected to a steering linkage isconnected to an output shaft equipped with a pinion. In a familiarmanner, it is possible to insert a torsionally elastic element, atorsion rod, for instance, between the two shafts. The pinion acts onthe toothed rack of a steering unit. The use of steering valves for suchsteering systems is familiar. For this purpose, a control pressureswitching unit is provided, which may, for instance, be a valve casingengaged with the output shaft and surrounding the input shaft. Hydraulicfluid is pumped through the system with a pump. In case of a twisting ofthe input shaft relative the valve casing, hydraulic pressure is fed toa hydraulic motor, which assists the rack motion in one of the twopossible directions.

Steering valves employed in power steering systems can be dividedroughly into two groups. One type of steering valve employed in powersteering systems is the steering valve with a so-called open center. Inthe neutral position of the steering valve with open center, hydraulicfluid at low pressure is pumped through the open valve into a reservoirby a pump. A twisting of the input shaft relative to the valve casingcauses an elevation of the pressure, which is then fed to theservomotor. The disadvantage of steering valves with open centers isthat a steady hydraulic flow must be maintained even in the neutralposition. Thus, wasted power must be produced by the drive motor,derived primarily from the pressure accumulation and the volume flow.

Such a steering valve, which has all the features of this class, apartfrom the open center, is disclosed in DE 43 17 818 C1. A reaction pistonwith an electrohydraulic converter EHW is employed as a torque/axialforce converter unit.

A power steering unit for passenger vehicles with a steering valvehaving an open center is known from DE 24 26 201 A1, wherein ahydraulic-bearing line is formed in the torsion element for purposes ofminimizing the size of the unit, but not with regard to wear and tear onthe seals.

Finally, DE 27 58 321 A1 discloses a steering valve with open center, inwhich an axial hole is drilled through the input shaft.

Another type of steering valve used in power steering systems is thesteering valve with a so-called closed center. The hydraulic flow to thetwo cylinder sides is interrupted in the neutral position of the valveby the closed center position. Thus, there is no hydraulic flow in theneutral position. An operating pressure, which, upon opening of thevalve only must be held within prescribed limits by the pump then goinginto action is applied to the practically closed valve by the pump.Because of the wasted power appearing for steering valves with opencenter even when the steering is not being operated, the steering valveswith closed center are superior in the neutral position when consideringpower. They have the disadvantage, however, that practically no directcorrelation exists between input torque and working pressure in thecylinder. In the prior art, therefore, additional measures were taken inorder to provide the operator with a relationship between the steeringforce to be input and the resulting hydraulic pressure. Conventionalseat valve arrangements are not without problems, since the seat valvesundergo a certain wear and require considerable manufacturing andassembly effort. Moreover, they confront the operator with a thresholdtorque.

A steering valve of the generic class with a closed center is known fromDE 28 34 421 A1. It has a torque/axial force converter unit and an axialborehole closed off by a seat valve for conducting hydraulic pressure.This additional seat valve not only increases the expense ofmanufacturing and assembly, it is also delicate and causes noise.

The special problem of steering valves with closed center with respectto the lack of a correlation between input torque and working pressurein the cylinder is expressed in steering initiation behavior, which isgenerally considered critical. Due to the fact that the hydraulic fluidis permanently present under maximum pressure at the control pressureswitching device, a very high dynamism in the operation of the controlpressure switching device results, since abrupt effects on systempressure occur.

Starting from this situation, the problem of the present invention is toimprove a steering valve with closed center of the generic type suchthat its response dynamics are considerably improved, such that it canbe made compact and reliable in its operation, and so that the steeringvalve can be simply and economically manufactured.

SUMMARY OF THE INVENTION

As a technical solution it is proposed that the steering valve have atleast one axial borehole for conducting hydraulic pressure which isclosed off at one end by a valve loaded against the hydraulic pressureby the torque/axial force converter unit.

The torque/axial force converter unit for steering valves with closedcenters has the effect that a torque exerted via the input shaftagainst, for instance, a resilient element is converted into a variationof axial force. A valve loaded by the converter unit against thehydraulic pressure thus does not open abruptly, but as a function of thetorque, of the spring characteristic of the resilient element and of thetorque/axial force converter. It is thus possible to improve responsebehavior considerably by regulating the pressure.

The valve is advantageously a seat valve. Such a seat valve can bringabout a complete sealing effect at one end of an axial borehole. Leakagelosses can be almost completely avoided. The torque/axial forceconverter unit advantageously has a control slide. A control slideoffers the possibility of loading the converter unit not only with theforce of a resilient element, but also with appropriately guidedhydraulic pressures. The control slide advantageously interacts with aspherical coupling to achieve a conversion between torque and axialforce. According to a proposal of the invention, the spherical couplinginteracts with a control slide interacting with the seat valve. Thecontrol slide is advantageously arranged in an axial borehole in theinput shaft. Loading of the control slide by a helical spring isachieved by the design according to the invention. One of the halves ofthe spherical coupling is correspondingly loaded by way of the controlslide. Balls are arranged between the two halves of the coupling, eachequipped with universal ball joints, such that upon introduction of atorque, the force of the helical spring must first be overcome in orderthat the two halves of the ball coupling can move apart due to the ballspressing out of the depressions. Advantageously, the helical spring isalso housed in the axial borehole arranged in the input shaft. Byintegrating the individual components of the converter unit into, forinstance, axial boreholes in the input shaft, a compact construction ofthe steering valve is guaranteed.

It is advantageously proposed that the seat valve be arranged with axialplay with respect to the reaction piston. The seat valve is arranged inan advantageous manner so as to be spring loaded in relation to thecontrol slide. Thus the seat valve can perform movements relative to thecontrol slide inside the axial play, independently of the valve positiondefined by torque hydraulic pressure relationships. After compensationfor the play, the pressure regulation is interrupted by the rigidcoupling and the maximum power assistance in the steering valve appears.

In an advantageous manner, a control space is formed in the axialborehole between the seat valve and the control slide. So long as theseat valve is closed, only tank pressure is present in the controlspace. Upon opening of the seat valve, a pressure which is fed viaappropriate lines, boreholes and the like to a hydraulic motor builds upin the control space.

The control space is connected in an advantageous manner to radialboreholes. The control pressure switching unit advantageously alsocomprises a valve casing surrounding the input shaft, so that the radialboreholes lead from the control pressure into the area between inputshaft and valve casing.

In an advantageous manner, the axial borehole is formed in a movableelement of the power steering valve. According to a proposal of theinvention, the axial borehole is guided axially through the outputshaft. Guiding the borehole axially through the input shaft is alsoproposed. In order to avoid leakage losses and save on the height of theunit, it is also proposed to guide the axial borehole through thetorsionally elastic element, which is practical to construct as atorsion rod.

The response dynamics of a steering valve with a closed center areconsiderably improved by the configuration according to the invention.For this purpose, an integrated pressure regulation system is proposed.By utilizing a seat valve interacting with an axial borehole, leakagelosses in the neutral position of the valve are avoided. This yields acost reduction in the manufacture of the valve since fewer componentsneed be used than for conventional valves. By coupling the seat valve tothe torque/axial force converter unit, a regulation of the cylinderpressure as a function of the angle of torsion of the input shaftresults. This makes it possible to adjust the valve characteristic curveeasily since the stiffness of the resilient element can be easilychanged. For a helical spring inserted in an axial borehole into theinput shaft, its spring deformation can be varied by adjusting thespring via an adjustment screw inserted into the borehole. The adjustingscrew interacts with a seal. Moreover, the converter unit represents acentering device for finding the midpoint. The respective cutoff pointof the valve characteristic curve can be adjusted by altering the freeaxial play between seat valve and control slide. The control slide inturn interacts with the spherical coupling.

By using a torsion tube, a hydraulic line accessible from the outsidecan be introduced through the torsion tube from the outer end into theoutput shaft as far as the input shaft. Seals that would otherwise beneeded between input and output shaft are thus unnecessary, which bringsabout a decisive reduction of friction. The torsion tube isadvantageously fastened in the output shaft at the touter end and isseated so that it can be twisted over a certain length into a boreholewith an inner diameter enlarged with respect to the outer diameter.While the torsion rods are typically arranged inside the input shaft inconventional steering valves, the input shaft! must have a minimumlength in order to guarantee the torsion characteristic of the torsionrod. By the arrangement according to the invention of the torsion tubeinside the output shaft, the input shaft can be correspondinglyshortened. Therefore the steering valve according to the invention has areduced constructive height with respect to conventional valves, whichrepresents a special feature of the present invention.

Due to the freedom from leakage achieved by using the seat valve, thepositive overlapping of the control edges can be reduced, whereby thedynamic properties of the valve are considerably improved. A specialadvantage of the valve according to the invention, finally, is that itis symmetrical on both control slides, that is, with respectively onlyone resilient element per steering side. Thus a sudden unilateralpressure buildup upon failure of the springs is out of the question.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and characteristics of the invention are seen fromthe description below on the basis of the figures. These show in

FIG. 1 a cross-sectional representation of an embodiment of the steeringvalve;

FIG. 2 a cross-sectional view along line A--A according to FIG. 1;

FIG. 3 a schematic side view of an embodiment of the torque/axial forceconverter;

FIG. 4 a cross-sectional view along line B--B according to FIG. 1; and

FIGS. 5a-5c diagrammatic representations of the valve characteristicssettings.

DETAILED DESCRIPTION OF THE INVENTION

The steering valve 1 represented in the figures comprises a housing 2,into which an input shaft 3 and an output shaft 4 are inserted by way ofbearings or seals 5,6. The input shaft 3 is enclosed by a valve casing7. The valve casing is rigidly connected to the output shaft 4. Theinput shaft 3 is connected to the output shaft 4 via the torsion tube 8,so that relative movement between input shaft 3 and output shaft 4 and,thus valve casing 7 as well, can occur. The output shaft 4 is equippedwith a pinion 9 which interacts with a rack of a steering system notshown in further detail.

A torque/axial force converter 10 is arranged in the vicinity of theupper end of the valve casing 7. As can be seen from FIGS. 2 and 3 aswell, the converter consists of two coupling disks 11,12, in whichso-called inclined planes 14 are formed into which balls 13 areinserted. If the one coupling disk is moved relative to the other, theballs 13, interacting with the inclined planes 14, cause the twocoupling disks 11,12 to be moved apart axially. Depending on the springload applied, a coupling is produced between the active torque and theaxial movement. In the illustrated embodiment, the coupling disk 12 isconnected to a pin 15 which in turn is axially movable in a radialborehole in the input shaft 3. The pin 15 is additionally connected to acontrol slide 16 arranged in a central axial borehole in the input shaft3 so that it can be displaced axially together with the pin 15. Theupper end of the control slide 16 is subjected to the action of ahelical spring 17 which is arranged between a set screw 18 closing offthe free end of the input shaft 3 and upper side of the control slide16. Moreover, a seal 19 is arranged between the helical spring 17 andthe set screw 18. The spring tension can thus be adjusted by way of theset screw 18.

The axial borehole 20, extending in the illustrated embodiment over theentire length of the steering valve 1 consists of the axial borehole 21,the axial borehole 28 in the torsion tube and the axial borehole 29 inthe input shaft. A hydraulic line 22 brought in from the outside up tothe steering valve 1 is connected via an annular articulated line 23 tothe axial borehole 20. For this purpose a radial borehole 24 is formedin the annular line 23. The connector piece is arranged in the annularline 23. Seals 25,26 close off the lower connector region with respectto the housing interior and the axial borehole. The hydraulic pressurecan thus be directed into the axial borehole 21 of the output shaft 4 inlowest area. Via radial boreholes 27 the pressure is additionallydirected into the axial borehole 28 in the torsion tube 8. The pressureis directed through the axial borehole 28 in the torsion tube up to theupper free end.

In the embodiment illustrated the diameter of the torsion tube 8 issmaller than the internal diameter of the axial borehole 21 in theoutput shaft, with the exception of the attachment region, in which theseal 25 is also formed. The torsional elasticity of the torsion tubeinside the output shaft can be achieved by these measures. In comparisonto conventional valves, therefore, the input shaft can be considerablyshortened, which is favorable to a shortening of the steering valve 1.

The free end of the torsion tube 8 is connected to the input shaft 3.For this purpose, the torsion tube is inserted into the axial borehole29 in the input shaft 3. At the same time, a seat valve 30 that isresiliently coupled to the control slide 16 is arranged in axialborehole 29 in the input shaft 3. In the illustrated embodiment, theconnection between the seat valve 30 and the control slide 16 consistsin a fastener with axial play 32, so that the seat valve 30 can be movedaxially relative to the control slide 16 within the scope of the play.Inside the play, the seat valve 30 is elastically coupled via the spring33. The space between the lower end of the control slide 16 and thespring plate of the seat valve 30 is the so-called control space, whichis connected via radial boreholes 35 to the control grooves 34 in thevalve casing 7. This is best seen from the cross section in FIG. 4.

Considered in purely mechanical terms, the illustrated steering valve 1functions in the following manner.

A torque acting on the input shaft 3 causes the coupling disk 12 of thetorque/axial force converter 10 to be moved via the vertical balls, withthe torque between the two coupling disks 11,12 opposing the force ofthe spring 17. Thus a middle centering is provided until the torqueexceeds the force of the spring 17, such that a relative twisting cantake place between the input shaft 3 and the output shaft 4, and thusthe valve casing 7. The decisive factor here is the torsional rigidityof the torsion tube 8.

The following function occurs hydraulically.

As long as there is no steering, that is, as long as no torque is actingon the input shaft 3, the two piston sides of the control slide 16 areconnected via the control boreholes to the tank, that is to say, theyare at tank pressure. The seat valve 30 closes off the axial borehole 28in the torsion tube free of leakage. This favors the formation ofsmaller positive overlaps in the vicinity of the control grooves,whereby the dynamic properties of the steering valve 1 are improved.Upon introduction of a torque via the input shaft, the above-describedmiddle centering of the valve, achieved primarily by the elastic forceof the spring 17 and the converter 10, is overcome. Subsequently thereis a relative twisting of the input shaft 3 with respect to the valvecasing 7, and above the response threshold of the steering valve theseat valve 30 is opened by the reduction of the tension of the spring 33and against the existing stored pressure. At the beginning of theopening process, a small volume flow flows into control space, in whichthe pressure now rises. The control pressure simultaneously correspondsas well to the back side of the seat valve and generates an axial forceproportional to the surface area of the pressure hole and the controlspace pressure. The pressure increases until the seat valve again closesand the original closing force is achieved. This means that a controlpressure is continuously adjusted, corresponding to a rotationalmovement of the input shaft into the cylinder, so that nodiscontinuities occur in the steering initiation range. The desiredvalve characteristic can be set by varying the stiffness of the spring33. Diagrams are shown in FIG. 5 in which the changes of pressure areplotted as a function of torque.

The helical spring 17 can be varied with the set screw 18, whereby themiddle centering shown in FIG. 5a can be displaced. The torque to beovercome, after which the seat valve 30 opens and changes in pressurecan be achieved, can be adjusted according to the set initial tension ofthe spring 17.

By variation of the spring stiffness of the spring 33 between controlslide 16 and seat valve 30, the slope of the change in pressure can beadjusted according to FIG. 5c. The torque input by an operator here isproportional to the respective cylinder pressure. Thus steeringbehaviors can be achieved that correspond to steering valves withhydraulic feedback. The restoring force of the power steering becomescomparable to a purely mechanical steering system.

In the parking range, it is desirable that the maximum manual force belimited by the system. After compensating for the play between seatvalve 30 and control slide 16, the coupling between the seat valve andthe control slide becomes rigid. The so-called cut-off can be varied byvariation of the play, as is shown in FIG. 5b. Upon reaching therespective setpoint, the pressure regulation is suppressed and themaximum system pressure builds up on the respective piston side.

The embodiment described above serves only to explain the invention,without limiting it to this embodiment form. In particular, the positionof the axial boreholes and the coupling of the closure valve to theconverter, as well as the embodiment of the converter, are variablewithin the scope of the invention.

    ______________________________________    List of reference symbols    ______________________________________    1     Steering valve 20    Axial borehole    2     Housing        21    Axial borehole, output shaft    3     Input shaft    22    Hydraulic line    4     Output shaft   23    Annular line    5     Bearing        24    borehole    6     Bearing        25    Seal    7     Valve casing   26    Seal    8     Torsion tube   27    Radial borehole    9     Pinion         28    Axial borehole, output shaft    10    Torque/axial force                         29    Axial borehole, input shaft          converter      30    Seat valve    11    Coupling disk  31    Vertical balls    12    Coupling disk  32    Fastener with axial play    13    Balls          33    Spring    14    Inclined plane 34    Control grooves    15    Pin            35    Radial boreholes    16    Control slide    17    Helical spring    18    Set screw    19    Seal    ______________________________________

We claim:
 1. Steering valve with closed center for supplying a controlelement with hydraulic pressure, consisting of an input shaft (3), anoutput shaft (4), a torsionally elastic element that is connected at oneend to the input shaft and at the other to the output shaft, a controlpressure switching device and a torque/axial force converter unit,characterized in that the steering valve (1) has at least one axialborehole (20) for carrying hydraulic pressure, which is closed at oneend by a valve loaded by the torque/axial force converter unit (10)against the hydraulic pressure.
 2. Steering valve according to claim 1,characterized in that the valve is a seat valve (30).
 3. Steering valveaccording to claim 1, characterized in that the torque/axial forceconverter unit (10) has a control slide (16).
 4. Steering valveaccording to claim 1, characterized in that the torque/axial forceconverter unit (10) comprises a spherical coupling.
 5. Steering valveaccording to claim 1, characterized in that the torque/axial forceconverter unit is connected to a control slide (16).
 6. Steering valveaccording to claim 5, characterized in that the control slide (16) isarranged in an axial borehole (29) in the input shaft (3).
 7. Steeringvalve according to one of claim 5, characterized in that the controlslide (16) is spring-loaded.
 8. Steering valve according to claim 7,characterized in that a screw (17) is inserted into the axial borehole(29) of the input shaft (3) in order to spring-load the control slide(16).
 9. Steering valve according to claim 1, characterized in that thevalve (30) is arranged on the control slide (16) by means of a fastenerwith axial play.
 10. Steering valve according to claim 9, characterizedin that the valve (30) is spring-loaded opposite the control slide (16).11. Steering valve according to claim 1, characterized in that a controlspace is formed in the vicinity of the valve (30) in the axial borehole(29) in the input shaft (3).
 12. Steering valve according to claim 11,characterized in that the control space is connected to radial boreholes(35).
 13. Steering valve according to claim 1, characterized in that thecontrol space is connected to the control pressure switching device. 14.Steering valve according to claim 1, characterized in that the controlpressure switching device comprises a valve casing (7).
 15. Steeringvalve according to claim 1, characterized in that the axial borehole isformed in one of the moving elements of the steering valve (1). 16.Steering valve according to claim 1, characterized in that the axialborehole is led axially through the output shaft (4).
 17. Steering valveaccording to claim 1, characterized in that the axial borehole is ledthrough the input shaft (3).
 18. Steering valve according to claim 1,characterized in that the axial borehole is led through the torsionallyelastic element.
 19. Steering valve according to claim 1, characterizedin that the torsionally elastic element is arranged in an axial borehole(21) in the output shaft (4) over the greatest portion of its length.20. Steering valve according to claim 1, characterized in that at leastone of the spring elements is adjustable.